I don't have intimate knowledge of the 4/4 (A Ford Crossflow 1600cc) , but in general the thickness of the wood used is not normally critical on non load-bearing parts. I think I used 3/8th Marine grade ply.
MMC certainly used odd bits and pieces , whatever was to hand, in the 1970s. It is all a lot more controlled now as each car gets the correct set of parts as they move down the line.
PS Welcome to TM
